Tail skid shock absorber and indicator

ABSTRACT

A tail skid shock absorber including an outer shock absorber canister, a crushable indicator cartridge disposed within the outer shock absorber canister, and an indicator rod coupled to the crushable indicator cartridge so as to move with a portion of the crushable indicator cartridge as a unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of and claims the benefit of U.S.Non-Provisional patent application Ser. No. 15/493,379 filed on Apr. 21,2017 (now U.S. Pat. No. 10,669,016 issued on Jun. 2, 2020), thedisclosure of which is incorporated by reference herein in its entirety.

BACKGROUND 1. Field

The aspects of the present disclosure generally relate to aircraft tailskids and aircraft incorporating those tail skids and, in particular, totail skids having energy absorption indicators that provide a usefullife remaining (e.g., serviceability) for an energy absorption unit ofthe tail skid.

2. Brief Description of Related Developments

An aircraft generally includes a tail skid that protects the aft portionor tail of the aircraft upon, for example, take off and or landing ofthe aircraft. For example, in some instances the tail skid will contact(e.g., a tail strike event) the runway, or other surface on which theaircraft is landing on or taking off from. In one aspect, the tail skidincludes energy absorption capabilities that absorb the contact energybetween the tail skid and, e.g., the runway. The energy absorptioncapabilities of the tail skid have a useful life, as the energyabsorption capabilities generally include a consumable crushablecartridge that is disposed within a tail skid shock absorber.

Currently, tail skids include indicator rods that provide informationregarding tail strike events. Generally, this tail strike eventinformation is provided by a series of two indicator rods disposed onthe tail skid shock absorber. These indicator rods are located within ahousing of a crush indicator system assembly that is separate from thecrushable cartridge of the tail skid, where the indicator rods are heldin a retracted position within the separate housing by shear pins. Theindicator rods have differing lengths so that, as the crushablecartridge is crushed, a portion of an exterior canister of the tail skidshock absorber pushes on at least one of the indicator rods due to, forexample, relative motion between portions of the exterior canister.Pushing on the at least one indicator rod shears the shear pin andcauses the indicator rod to fall freely within the separate housing ofthe crush indicator system, so that a portion of the indicator rod isexposed outside of the tail skid shock absorber to visually indicatethat the energy absorption capability of the crush cartridge aspartially or entirely exhausted. Currently, after being crushed, thetail skid shock absorber extends and returns to its previous length toproperly stow during flight. This objective requires the indication tobe decoupled from the outer housing. For example, if the tail strikeevent is of at least a first magnitude, a first indicator rod freelyfalls to a fully deployed position so as to be visually exposed toprovide a visual indicator that the crushable cartridge still has aboutfull energy absorption available; while if the tail strike event is ofat least a second magnitude (which is greater than the first magnitude),a second indicator rod freely falls to a fully deployed position to bevisually exposed along with the first indicator rod to provide a visualindicator that the crushable cartridge has less than full energyabsorption available. As described above, with conventional tail skidenergy absorption indication systems, the amount of energy absorptionavailable in the crushable cartridge is indicated in a very coarsemanner (e.g., about full capability and less than full capability).

Further, the indicator rods of the conventional tail skid energyabsorption indication systems are subject to being dislodged from thetail skid shock absorber. For example, as the indicator rods are allowedto freely fall so as to protrude from the tail skid shock absorber, theindicator rods are free to move relative to the tail skid shockabsorber. These indicator rods are only held in place by a small head onthe respective indicator rod. However, movement of the indicator rodsmay cause a hole to open up or expand in the indicator assembly throughwhich the indicator rods extend, thereby allowing the indicator rod tofall entirely through the hole and onto the ground. Further, theindicator rods are exposed such that tail skid components may causebending of the indicator rods. The bending of the rods may also causethe indicator rods to be removed or dislodged from the indicatorassembly. The purpose of the crush indicator system is to provide anindication of useful life of the tail skid shock absorber after an eventthat does not exceed the second magnitude. However, once exposed ortriggered, the indicator rod is exposed for many additional flights andis subject to being damaged (e.g., falling to the ground or being bent).

As can be seen above, because the indicator rods of the conventionalcrush indicator system are a separate unit from the canister, theindicator rods, or some other portion of the indicator system, may bedislodged and result in debris on the runway (or some other place on theground). Further, conventional indicator rods are only triggered basedon exceeding a first or second threshold, so as to shear the shear pinsholding the first and second indicator rods in place within the separatehousing, where the indicator rods are fully deployed without anyintermediate position. Thus, the conventional crush indicator systemdoes not provide any indication of the magnitude of the tail strikeevent other than exceeding one or both thresholds.

SUMMARY

The following is a non-exhaustive list of examples, which may or may notbe claimed, of the subject matter according to the present disclosure.

One example of the subject matter according to the present disclosurerelates to a tail skid shock absorber comprising an outer shock absorbercanister; a crushable indicator cartridge disposed within the outershock absorber canister; and, an indicator rod coupled to the crushableindicator cartridge, so as to move with a portion of the crushableindicator cartridge as a unit.

Another example of the subject matter according to the presentdisclosure relates to a crushable indicator cartridge for a tail skidshock absorber, the crushable indicator cartridge comprising anindicator rod; an elongated member defining a crush core; a first platecoupled to the crush core, the indicator rod being coupled to the firstplate; and, a second plate coupled to the crush core, the second platedefining an outer surface of the crushable indicator cartridge andincluding a second plate aperture through which the indicator rodextends.

Still another example of the subject matter according to the presentdisclosure relates to an aircraft comprising an airframe; and a tailskid shock absorber coupled to the airframe, the tail skid shockabsorber including an outer shock absorber canister; a crushableindicator cartridge disposed within the outer shock absorber canister;and, an indicator rod coupled to the crushable indicator cartridge so asto move with a portion of the crushable indicator cartridge as a unit.

Yet another example of the subject matter according to the presentdisclosure relates to a method for indicating an amount of energyabsorption remaining in a tail skid shock absorber, the methodcomprising inserting a crushable indicator cartridge into an outer shockabsorber canister, where the crushable indicator includes an indicatorrod coupled to the crushable indicator cartridge, so as to move with aportion of the crushable indicator cartridge as a unit; and, crushingthe crushable indicator cartridge through contact of the airplane tailskid with an object, so that the indicator rod extends and is exposedfrom the outer shock absorber canister.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described examples of the present disclosure in generalterms, reference will now be made to the accompanying drawings, whichare not necessarily drawn to scale, and wherein like referencecharacters designate the same or similar parts throughout the severalviews, and wherein:

FIG. 1 is a schematic illustration of an exemplary aircraft inaccordance with aspects of the present disclosure;

FIG. 2 is a schematic illustration of a tail skid in accordance withaspects of the present disclosure;

FIG. 3A is a schematic cross-sectional illustration of a tail skid shockabsorber in accordance with aspects of the present disclosure;

FIG. 3B is a schematic cross-sectional illustration of a portion of thetail skid shock absorber of FIG. 3A in accordance with aspects of thepresent disclosure;

FIG. 3C is a schematic illustration of a portion of the tail skid shockabsorber of FIG. 3A in accordance with aspects of the presentdisclosure;

FIG. 4A is a schematic illustration of a portion of a crushableindicator cartridge of the tails skid shock absorber of FIG. 3A inaccordance with aspects of the present disclosure;

FIG. 4B is a schematic illustration of a portion of the crushableindicator cartridge of FIG. 4A in accordance with aspects of the presentdisclosure;

FIG. 5A is a schematic illustration of a portion of the tail skid shockabsorber of FIG. 3A in accordance with aspects of the presentdisclosure;

FIG. 5B is a schematic illustration of a portion of the tail skid shockabsorber of FIG. 3A in accordance with aspects of the presentdisclosure;

FIG. 6A is a schematic illustration of a portion of the aircraft of FIG.1 including the tail skid of FIG. 2 in accordance with aspects of thepresent disclosure;

FIG. 6B is a schematic illustration of a portion of the tail skid ofFIG. 2 including the tail skid shock absorber of FIG. 3A in accordancewith aspects of the present disclosure;

FIG. 6C is a schematic illustration of a portion of the tail skid ofFIG. 2 including the tail skid shock absorber of FIG. 3A in accordancewith aspects of the present disclosure;

FIG. 6D is a schematic illustration of a portion of the tail skid ofFIG. 2 including the tail skid shock absorber of FIG. 3A in accordancewith aspects of the present disclosure;

FIG. 7A is a schematic cross-sectional illustration of the tail skidshock absorber of FIG. 3A in accordance with aspects of the presentdisclosure;

FIG. 7B is a schematic cross-sectional illustration of the tail skidshock absorber of FIG. 3A in accordance with aspects of the presentdisclosure; and

FIG. 8 is a flow diagram of a method in accordance with aspects of thepresent disclosure.

DETAILED DESCRIPTION

Referring to FIGS. 1, 2, 3A and 3B, the aspects of the presentdisclosure described herein provide a tail skid shock absorber 110 thatincludes a single indicator rod 310 that is integral with the tail skidshock absorber 110. For example, the indicator rod 310 is retained by acrushable indicator cartridge 300, which defines a crush core 302,located within an outer shock absorber canister 200 of the tail skidshock absorber 110. As the crushable indicator cartridge 300 is crushed,the indicator rod 310 moves a unit with a portion of the crushableindicator cartridge 300, so that a portion of the indicator rod 310extends outside of the outer shock absorber canister 200. Extension ofthe indicator rod 310 outside of the outer shock absorber canister 200provides a visual indicator of an amount of energy absorption remainingin the crushable indicator cartridge 300. The aspects of the presentdisclosure described herein provide for a reduced number of parts to beordered when the crushable indicator cartridge 300 is replaced. Thisreduced number of parts is due to, for example, the integration of theindicator rod 310 with the crush core 302 that is defined by thecrushable indicator cartridge 300. Accordingly, the aspects of thepresent disclosure eliminate the separate crush indicator systemsdescribed above in the background section. In addition, in aspects ofthe present disclosure, the indicator rod 310 is held captive orretained by the crush core 302 of the crushable indicator cartridge 300.Retention of the indictor rod 310 by the crush core 302 substantiallyprevents the indicator rod 310 from being dislodged from the tail skidshock absorber 110 when the indicator rod 310 is exposed from the outershock absorber canister 200 for visual inspection. The retention of theindicator rod 310 by the crush core 302 may also provide an indicationof the magnitude of a tail strike event because the amount of forcerequired to crush the crush core 302 a predetermined distance may beknown and can be determined by, for example, measuring an amount of theindicator rod 310 that is exposed for visual inspection. In one aspect,the indicator rod 310 includes suitable graduations 310GR (see FIGS. 3Cand 4B) configured to provide a measurement scale with respect to anamount of exposure of the indicator rod 310. The aspects of the presentdisclosure also provide a recessed area 350RA, 350RB in which theindicator rod 310 extends for visual inspection to shield the indicatorrod 310 from contact with other objects.

Illustrative, non-exhaustive examples, which may or may not be claimed,of the subject matter according to the present disclosure are providedbelow.

Still referring to FIGS. 1, 2, 3A and 3B as well as FIGS. 5A and 5B, thetail skid shock absorber 110 includes an outer shock absorber canister200. The outer shock absorber canister 200 includes an elongated tube500 and at least one end fitting 350A where the end fitting isconfigured to couple with one of a tail skid shoe 270 and an airframe100F of the aircraft 100. For example, the elongated tube 500 includes afirst tube portion 500A and a second tube portion 500B that are movablerelative to each other along a longitudinal axis LAX of the elongatedtube 500 (e.g., the shock absorber canister 200). In one aspect, thesecond tube portion 500B may be slidable within the first tube portion500A. The first tube portion 500A and the second tube portion 500Binclude any suitable locking features 500L1, 500L2 to preventover-extension of the first tube portion 500A and the second tubeportion 500B in direction 700B which may cause separation of the firsttube portion 500A from the second tube portion 500B.

The first tube portion 500A and the second tube portion 500B form anexterior of the shock absorber canister 200. In one aspect, the secondtube portion 500B is integrally formed with an end fitting 350B; whilethe end fitting 350A is removably coupled to the first tube portion 500Aof the elongated tube 500 in any suitable manner. For example, the firsttube portion 500A includes a plurality of lugs 501A-501F and the endfitting 350A includes a plurality of mating lugs 502A-502F. The lugs501A-501F and the mating lugs 502A-502F are radially spaced about thelongitudinal axis LAX so that the mating lugs 502A, 502F are coupled anddecoupled with the lugs 501A-501F through rotation of the end fitting350A relative to the first tube portion 500A. In one aspect, anysuitable anti-rotation device 550 is provided to prevent rotation anddisassembly/decoupling of the end fitting 350A and the first tubeportion 500A as will be described in greater detail herein. In otheraspects, the end fitting 350A may be coupled to first tube portion inany suitable manner such as by a threaded coupling, a pinned coupling, aclipped coupling, etc.

In the aspect illustrated in FIG. 2, the end fitting 350A is illustratedas being coupled to the tail skid shoe 270. The end fitting 350B isillustrated as being coupled to the airframe 110F through, e.g., tailskid linkage 290. In other aspects the end fitting 350A may be coupledto the airframe 110F while the end fitting 350B is coupled to the tailskid shoe 270.

Referring now to FIGS. 3A, 3B, 4A and 4B, the tail skid shock absorber110 also includes a crushable indicator cartridge 300 disposed withinthe outer shock absorber canister 200. An outer diameter 300D of thecrushable indicator cartridge 300 is smaller than an inner diameter 500Dof the outer shock absorber canister 200 to allow insertion and removalof the crushable indicator cartridge 300 to and from the outer shockabsorber canister 200. The crushable indicator cartridge 300 includes afirst end 300E1 and a second end 300E2. The crushable indicatorcartridge 300 may be constructed of any suitable material, such as anysuitable corrugated metal and/or composite materials, where thecrushable indicator cartridge has predetermined crush characteristics,e.g., to begin compaction/crushing at a predetermined load. Thecompression strength of the crushable indicator cartridge 300 is knownthroughout a predetermined crushable distance D (see FIG. 7B) of thecrushable indicator cartridge 300. The crushable indicator cartridge 300has an uncompressed length D1 and forms an elongated member 302M thatdefines a crush core 302. In one aspect, the crush core 302 includes afirst portion Z1 at, for example, the second end 300E2, that has areduced crushed capability compared to a second portion Z2 of the crushcore 302. For example, the crush core 302 may include a channel 302CHextending through the crush core 302 between the first end 300E1 and thesecond end 300E2, where the channel 302CH has a first diameter 300ID1within the first portion Z1 and a second diameter 300ID2 through asecond portion Z2 (FIG. 3A) of the crush core 302. As can be seen inFIG. 3B, the change from the first diameter 300ID1 to the seconddiameter 300ID2 forms a step or seating surface SS.

In one aspect, the crush core 302 includes a first plate 320, a secondplate 325 and a single indicator rod 310. The first plate 320, thesecond plate 325 and the single indicator rod 310 are coupled to thecrush core 302, so as to be integral to and form the crushable indicatorcartridge 300. For example, the first plate 320 is coupled to theseating surface SS where coupling the first plate 320 to the seatingsurface SS captures the indicator rod 310 between the first plate 320and the seating surface SS. In one aspect, the first plate 320 includesa first plate aperture 320A and the indicator rod 310 includes a rodhead 310H. The indicator rod 310 is inserted through the first plateaperture 320A so as to extend from the first plate 320 where the rodhead 310H cooperates with the first plate aperture 320A so that theindicator rod 310 is retained on the first plate 320. In one aspect, thefirst plate aperture 320A includes a recess 320AR in which the rod head310H is disposed such that coupling of the first plate 320 to theseating surface SS sandwiches or otherwise captures the rod head 310Hbetween the first plate 320 and the seating surface SS. The indicatorrod 310 moves as a unit with a portion of the crushable indicatorcartridge 300, such as the seating surface SS and first plate 320, asthe crush core 302 is crushed. In other aspects, the indicator rod 310may be coupled to the first plate 320 in any suitable manner such asthrough any suitable chemical or mechanical fastener. The first plate320 may be coupled to the seating surface SS in any suitable manner suchas by any suitable chemical or mechanical fasteners. The second plate325 is coupled to the second end 300E2 in any suitable manner, such aswith any suitable chemical or mechanical fasteners and defines an outersurface 300S of the crushable indicator cartridge 300. The second plate325 includes a second plate aperture 325A and is aligned with the firstplate 320, so that the indicator rod 310 extends through both the firstplate aperture 320A and the second plate aperture 325A.

In one aspect, referring also to FIGS. 5A and 5B, the second plate 325includes fasteners 400A, 400B for coupling the second plate 325, andhence the crushable indicator cartridge 300, to the end fitting 350A ofthe tail skid shock absorber 110. Coupling of the crushable indicatorcartridge 300 to the end fitting 350A facilitates the insertion andremoval of the crushable indicator cartridge 300 to and from the outershock absorber canister 200. Coupling the crushable indicator cartridge300 to the end fitting 350A also facilitates the coupling of the endfitting 350A and the crushable indicator cartridge 300 to the outershock absorber canister 200 as a single unit. This is particularlyuseful when removing a crushed crushable indicator cartridge 300. Forexample, decoupling of the end fitting 350A from the outer shockabsorber canister 200 also pulls the crushed crushable indicatorcartridge 300 out from inside the outer shock absorber canister 200making replacement of the crushable indicator cartridge 300 easier andfaster to accomplish compared to a crushable indicator cartridgeuncoupled from the end fitting and lacking any grasping locations fromwhich the crushable indicator cartridge may be handled for removal fromthe outer shock absorber canister 200. In one aspect, the end fitting350A includes at least one end fitting aperture 351A, 351B, throughwhich the indicator rod 310 extends The end fitting 350A also includesfastener apertures 401A, 401B. The fasteners 400A, 400B of the crushableindicator cartridge 300 and the fastener apertures 401A, 401B of the endfitting 350 are aligned with each other such that the at least one endfitting aperture 351A, 351B is aligned with the indicator rod 310 whenthe end fitting 350A is coupled to the crushable indicator cartridge300. In one aspect, the end fitting 350A includes two end fittingapertures 351A, 351B that are symmetrically arranged relative tolongitudinal axis LAX and the fastener apertures 401A, 401B. Thesymmetrically arranged end fitting apertures 351A, 351B ensure one ofthe end fitting apertures 351A, 351B is aligned with the indicator rod310 upon coupling of the end fitting 350A to the crushable indicatorcartridge 300 independent of a rotational orientation of the end fitting350A about the longitudinal axis LAX (e.g., the fasteners 400A, 400B andthe fastener apertures 401A, 401B align one of the end fitting apertures351A, 351B with the indicator rod 310).

As described above, any suitable anti-rotation device 550 is provided toprevent rotation and disassembly/decoupling of the end fitting 350A andthe first tube portion 500A of the outer shock absorber canister 200.For example, the anti-rotation device 550 may be coupled to both thefirst tube portion 500A (e.g., the elongated tube 500) and the endfitting 350A. For example, the anti-rotation device 550 may beconfigured to couple with the end fitting 350A through one of thefasteners 400A, 400B and be configured to couple with the first tubeportion 500A through an interface with one or more of the lugs501A-501F. For example, the fasteners 400A, 400B may be arranged on theend fitting 350A, so as to be radially spaced between a respectiveadjacent pair of the mating lugs 502A-502F. For example, fastener 400Ais radially spaced between adjacent mating lugs 502E and 502F whilefastener 400B is radially spaced between adjacent mating lugs 502B,502C. As such, when the end fitting 350A and the first tube portion 500Aare coupled the fasteners 400A, 400B are also aligned with recesses501RR formed by and between adjacent lugs 501A-501F. The anti-rotationmember 550 cooperates with both a respective fastener 400A, 400B and arespective recess 501RR to prevent relative rotation between the endfitting 350A and the first tube portion 500A. The anti-rotation device550 is removably coupled to end fitting 350A, such as with one of thefasteners 400A, 400B, so that the anti-rotation device 550 is removed toallow rotation of the end fitting 350A for removal of the end fitting350A from the first tube portion 500A.

In one aspect, referring to FIGS. 3A, 3B, 4A and 4B, the tail skid shockabsorber 110 includes fluid drainage, so that any fluids located withinan interior of the tail skid shock absorber 110 are drained from theinterior to an exterior of the tail skid shock absorber 110. Forexample, the channel 302CH of the crushable indicator cartridge directsfluids, such as by gravity, towards end 300E2, noting that both thefirst plate 320 and the second plate 325 include respective channelapertures 320CA, 325CA through which the fluid flows. The second plate325 also includes a fluid drainage groove 450 on the outer surface 300Sof the crushable indicator cartridge 300 that bi-directionally extendsradially from the channel aperture 325CA. In one aspect, the fluiddrainage groove 450 is radially aligned with the second plate aperture325A through which the indicator rod 310 extends. Alignment of the fluiddrainage groove 450 with the second plate aperture 325A directs fluid tothe fastener apertures 401A, 401B of the end fitting 350A where thefastener apertures 401A, 401B serve as drainage holes to evacuate thefluid from the interior of the tail skid shock absorber 110.

Referring now to FIGS. 4B, 5A, 5B and 6D, the end fitting 350A includesa recessed area 350RA adjacent each of the end fitting apertures 351A,351B that is shaped and sized so as to partially surround the indicatorrod 310 when the indicator rod 310 extends through one of the endfitting apertures 351A, 351B to provide a visual indication of an amountof remaining energy absorption in the crushable indicator cartridge 300.The recessed area 350RA partially surrounds the indicator rod 310 toshield or otherwise protect the indicator rod 310, as it extends fromone of the end fitting apertures 351A, 351B, from debris and contactwith objects that may cause bending or other damage to the indicator rod310. In one aspect, the end fitting apertures 351A, 351B and therecessed area 350RA may be disposed on the end fitting 350A, so that theindicator rod 310 is positioned on a trailing side TS of the tail skidshock absorber 110 relative to forward movement of the aircraft 100 tofurther protect the indicator rod 310 from contact with debris or otherobjects.

In one aspect, referring also to FIGS. 3B, 3C, 4B, 5A, 6C, 6D, 7A and7B, the end fitting 350A includes a datum surface 352 adjacent the endfitting aperture 351A, 351B where the datum surface 352 defines ameasuring point from which a distance X1, X2 the indicator rod 310protrudes from the end fitting aperture 351A, 351B is determined. Forexample, the distance X1, X2 may be measured so that a magnitude of thetail strike event can be determined using the known crushcharacteristics of the crush core 302 of the crushable indicatorcartridge 300. In other aspects, the indicator rod 310 includesgraduations 310GR that indicate an amount of remaining energy absorptionof the tail skid shock absorber 110. For example, the surface 310S (seeFIGS. 4B and 5A) of the indicator rod 310 may be divided into one ormore sections, such as for example, a first section 310S1, a secondsection 310S2 and a third section 310S3. Here the third section 310S3 isproximate the head 310H, the first section 310S1 is distal from the head310H and the second section 310S2 is disposed between the first section310S1 and the third section 310S3. In this aspect, the second section310S2 comprises a graduated region 310GR which may be knurled, beadblasted, painted, textured or otherwise provide a visual demarcation orgraduation from each of the first section 310S1 and the third section310S3. In one aspect, the visual demarcation or graduation may include aseries of graduated lines that indicate the amount of extension of theindicator rod. The first section 310S1, the second section 310S2 and thethird section 310S3 of the indicator rod 310 may be color coded toprovide for easy visual inspection of the crushable indicator cartridge300. For example, the first section 310S1 may have a green colorindicating no replacement of the crushable indicator cartridge 300 isnecessary. The second section 310S2 may have a yellow color indicatingthat some energy absorption has occurred and that the crushableindicator cartridge 300 is nearing replacement. The third section 310S3may have a red color indicating replacement of the crushable indicatorcartridge 300 is necessary.

The position of the demarcations between the second section 310S2 andeach of the first section 310S1 and the third section 310S3 along theindicator rod 310, in one aspect, depends on and is related to thepredetermined crush characteristics of the crush core 302 of thecrushable indicator cartridge 300 to provide a visual indication of theamount of energy absorption remaining in the tail skid shock absorber110. For example, as illustrated in FIG. 6B, the indicator rod 310 (seealso FIGS. 3C, 6C and 6D) provides a visual indication that there issubstantially a full amount of energy absorption available in thecrushable indicator cartridge 300 where the distal end 310DE of theindicator rod 310 is substantially even with the datum surface 352 (orthe distal end or free end 310DE of the indicator rod 310 is disposedwithin the end fitting aperture 351A, 351B—see also FIG. 3B). As can beseen in FIGS. 6C and 7A, at least a portion of the first section 310S1extends from the end fitting aperture 351A, 351B (but the second section310S2 (see FIGS. 3C and 6D) is not yet extending past the datum surface352). Extension of at least the portion of the first section 310S1 fromthe end fitting aperture 351A, 351B provides a visual indication thatsome crush of the crush core 302 occurred, but that the crushableindicator cartridge 300 remains operational (e.g., the crushableindicator cartridge 300 has sufficient energy absorption remaining toprotect against a tail strike event, i.e., the crushable indicatorcartridge 300 remains serviceable). For example, full visual exposure ofthe first section 310S1 from the end fitting aperture 351A, 351B (seealso FIG. 3B), with the second section 310S2 (see FIGS. 3C and 6D) notyet extending past the datum surface 352, indicates the crush core 302has been crushed by an amount X1 in direction 700A relative to the endfitting 350A, so that the crush core 302 has a length D2. It is notedthe amount of crush of the crushable indicator cartridge 300 indirection 700A, relative to the end fitting 350A, may be in directcorrelation to the amount of movement of the indicator rod 310 due tothe first plate 320, the seating surface SS of the crush core 302 andthe indicator rod 310 moving as a unit as the crush core 302 is crushed.As can be seen in FIG. 7A, after being crushed an amount X1, the crushcore 302 still has the ability to be crushed a distance D-X1.

As can be seen in FIGS. 6D and 7B, further crushing of the crush core302 from subsequent tail strike events may cause further movement of theindicator rod 310 in direction 700A, relative to the end fitting 350A(or movement of the end fitting 350A in direction 700B relative to theindicator rod 310), so that some or all of the second section 310S2 isvisually exposed through the end fitting aperture 351A, 351B. Forexample, full visual exposure of the second section 310S2 from the endfitting aperture 351A, 351B, with or without the third section 310S3(see FIG. 3C) extending past the datum surface 352, indicates the crushcore has been crushed by an amount X2 in direction 700A relative to theend fitting 350A. It is again noted that the amount of crush of thecrush core 302 may be in direct correlation to the amount of movement ofthe indicator rod 310 due to the first plate 320, the seating surface SSof the crush core 302 and the indicator rod 310 moving as a unit as thecrush core 302 is crushed. As can be seen in FIG. 7B, the crush core 302has been crushed to a length D3, which may result in substantially allof the crush/energy absorption capability of the crushable indicatorcartridge 300 being exhausted.

Referring now to FIGS. 2, 3A, 5B, 6A-6D, 7A, 7B and 8 an exemplaryoperation of the tail skid shock absorber 110 will be provided. In oneaspect, the crushable indicator cartridge 300 is inserted into the outershock absorber canister 200 (FIG. 8, Block 800). For example, asdescribed above the crushable indicator cartridge 300 is coupled to theend fitting 350A using the fasteners 400A, 400B so that the end fitting350A and the crushable indicator cartridge 300 are coupled to the outershock absorber canister 200 as a single one piece unit. In one aspect,the crushable indicator cartridge 300 may be crushed to visibly exposethe indicator rod 310 through the end fitting aperture 351A, 351B (FIG.8, Block 810). For example, crushing of the crushable indicatorcartridge 300 may occur when the tail skid shoe 270 contacts the runwayproducing a tail strike force F that pushes the first tube portion 500Aof the tail skid shock absorber 110 in direction 700B to cause relativemovement between the first tube portion 500A and the second tube portion500B of the tail skid shock absorber 110, noting that the second tubeportion 500B may be held stationary relative to the airframe 110F by thetail skid linkage 290 such that the first tube portion 500A movesrelative to the second tube portion 500B. As can be seen in FIGS. 3A, 7Aand 7B the crushable indicator cartridge 300 abuts the second tubeportion 500B at the first end 300E1 of the crushable indicator cartridge300, so that the position of the crushable indicator cartridge 300 isalso held stationary relative to the airframe 100F and the second tubeportion 500B. As such, movement of the first tube portion 500A indirection 700B causes crushing of the crushable indicator cartridge 300beginning with the first portion Z1 (see FIG. 3A) of the crush core 302and then with subsequent crushing of the crush core 302 within thesecond portion Z2 (see FIG. 3A), noting that there may be simultaneouscrushing of the crush core 302 within the first portion Z1 and withinthe second portion Z2 during relative movement between the first tubeportion 500A and the second tube portion 500B.

As described above, the relative movement between the first tube portion500A and the second tube portion 500B causes crushing of the crushableindicator cartridge 300 and visually exposes the indicator rod 310through the end fitting aperture 351A, 351B. The visual exposure of theindicator rod 310 indicates the amount of energy absorption remaining inthe crushable indicator cartridge 300 (FIG. 8, Block 820). For example,a pilot or maintenance personnel performing a pre-flight check mayvisually observe the exposure of the indicator rod 310 from the groundduring the pre-flight check to verify how much of the indicator rod 310is visually exposed and to determine if the crushable indicatorcartridge 300 needs to be replaced. In one aspect, the amount ofexposure or extension of the indicator rod 310 relative to, for examplethe datum surface 352 may provide for the determination of a magnitudeof a tail strike event as described above. Where a predetermined amountof the indicator rod 310 is visually exposed (such as all or a portionof the second section 310SS being visually exposed) the crushableindicator cartridge is replaced (FIG. 8, Block 830). As described above,coupling of the crushable indicator cartridge 300 with the end fitting350A provides for removal/decoupling of the crushable indicatorcartridge 300 and the end fitting 350A from the outer shock absorbercanister 200 as a unit, making it easy to pull the crushed crushableindicator cartridge 300 from the outer shock absorber canister 200. Oncethe crushed crushable indicator cartridge 300 is removed, the first tubeportion 500A and the second tube portion 500B may be pulled indirections 700A and 700B to return the outer shock absorber canister 200to an uncompressed length (corresponding to the uncompressed length D1of the crushable indicator cartridge 300) for insertion of replacementor uncompressed crushable indicator cartridge 300 into the outer shockabsorber canister 200.

As can be seen from the above description, the aspects of the presentdisclosure provide a tail skid shock absorber 110 that includesprogressive crush indication by which pilots, maintenance personnel orother individuals can visually, from the ground, determine continuedserviceability of the tail skid shock absorber 110 in the event of atail strike event. As can also be seen from the above-description, theindicator rod 310 is held captive within the crushable indicatorcartridge (and is protected from debris) to substantially prevent theindicator rod 310 from being dislodged from the trail skid shockabsorber 110.

The following are provided in accordance with the aspects of the presentdisclosure:

A1. A tail skid shock absorber comprising:

an outer shock absorber canister;

a crushable indicator cartridge disposed within the outer shock absorbercanister; and

an indicator rod coupled to the crushable indicator cartridge so as tomove with a portion of the crushable indicator cartridge as a unit.

A2. The tail skid shock absorber of paragraph A1, wherein the crushableindicator cartridge forms an elongated member that defines a crush coreincluding:

a first plate coupled to the crush core, the indicator rod being coupledto the first plate; and

a second plate coupled to the crush core, the second plate defining anouter surface of the crushable indicator cartridge and including asecond plate aperture through which the indicator rod extends.

A3. The tail skid shock absorber of paragraph A2, wherein the indicatorrod includes a rod head and the first plate includes a first plateaperture, the indicator rod extends through the first plate aperturewhere the rod head cooperates with the first plate aperture so that theindicator rod is retained on the first plate.

A4. The tail skid shock absorber of paragraph A2, wherein the secondplate includes fasteners configured to couple the crushable indicatorcartridge to an end fitting of the tail skid shock absorber.

A5. The tail skid shock absorber of paragraph A2, wherein the secondplate includes a fluid drainage groove.

A6. The tail skid shock absorber of paragraph A1, wherein the outershock absorber canister includes an elongated tube and an end fitting,the end fitting being configured to couple with one of a tail skid shoeand an airframe, where the end fitting includes at least one end fittingaperture through which the indicator rod extends.

A7. The tail skid shock absorber of paragraph A6, wherein the endfitting includes a datum surface adjacent the end fitting aperture wherethe datum surface defines a measuring point from which a distance theindicator rod protrudes from the end fitting aperture is determined.

A8. The tail skid shock absorber of paragraph A6, wherein the indicatorrod extends through the end fitting aperture to provide a visualindicator of an amount of remaining energy absorption in the crushableindicator cartridge.

A9. The tail skid shock absorber of paragraph A6, wherein the elongatedtube includes a first tube portion and a second tube portion that aremoveable relative to each other along a longitudinal axis of theelongated tube.

A10. The tail skid shock absorber of paragraph A6, wherein the endfitting is removably coupled to the elongated tube so that removal ofthe end fitting from the elongated tube facilitates replacement of thecrushable indicator cartridge.

A11. The tail skid shock absorber of paragraph A6, further comprising ananti-rotation member removably coupled to both the elongated tube andthe end fitting.

A12. The tail skid shock absorber of paragraph A6, wherein the at leastone end fitting aperture includes two end fitting aperturessymmetrically arranged on the end fitting.

A13. The tail skid shock absorber of paragraph A1, wherein an outerdiameter of crushable indicator cartridge is smaller than an innerdiameter of the outer shock absorber canister.

A14. The tail skid shock absorber of paragraph A1, wherein the indicatorrod includes at least one graduation that indicates an amount ofremaining energy absorption of the tail skid shock absorber.

B1. A crushable indicator cartridge for a tail skid shock absorber, thecrushable indicator cartridge comprising:

an indicator rod;

an elongated member defining a crush core;

a first plate coupled to the crush core, the indicator rod being coupledto the first plate; and

a second plate coupled to the crush core, the second plate defining anouter surface of the crushable indicator cartridge and including asecond plate aperture through which the indicator rod extends.

B2. The crushable indicator cartridge of paragraph B1, wherein theindicator rod includes a rod head and the first plate includes a firstplate aperture, the indicator rod extends through the first plateaperture where the rod head cooperates with the first plate aperture sothat the indicator rod is retained on the first plate.

B3. The crushable indicator cartridge of paragraph B1, wherein theindicator rod and the first plate move as a single unit along alongitudinal axis of the elongated member as the crush core is crushed.

B4. The crushable indicator cartridge of paragraph B1, wherein theindicator rod includes graduations that indicate an amount of remainingenergy absorption of the tail skid shock absorber.

B5. The crushable indicator cartridge of paragraph B1, wherein theelongated member is configured for insertion to and removal from anouter shock absorber canister of a tail skid shock absorber such that anouter diameter of the elongated member is smaller than an inner diameterof the outer shock absorber canister.

B6. The crushable indicator cartridge of paragraph B1, wherein thesecond plate includes fasteners configured to couple the crushableindicator cartridge to an end fitting of a tail skid shock absorber.

B7. The crushable indicator cartridge of paragraph B1, wherein thesecond plate includes a fluid drainage groove.

C1. An aircraft comprising:

an airframe; and

a tail skid shock absorber coupled to the airframe, the tail skid shockabsorber including

an outer shock absorber canister;

a crushable indicator cartridge disposed within the outer shock absorbercanister; and

an indicator rod coupled to the crushable indicator cartridge so as tomove with a portion of the crushable indicator cartridge as a unit.

C2. The aircraft of paragraph C1, wherein the crushable indicatorcartridge forms an elongated member that defines a crush core including:

a first plate coupled to the crush core, the indicator rod being coupledto the first plate; and

a second plate coupled to the crush core, the second plate defining anouter surface of the crushable indicator cartridge and including asecond plate aperture through which the indicator rod extends.

C3. The aircraft of paragraph C2, wherein the indicator rod includes arod head and the first plate includes a first plate aperture, theindicator rod extends through the first plate aperture where the rodhead cooperates with the first plate aperture so that the indicator rodis retained on the first plate.

C4. The aircraft of paragraph C2, wherein the second plate includesfasteners configured to couple the crushable indicator cartridge to anend fitting of the tail skid shock absorber.

C5. The aircraft of paragraph C2, wherein the second plate includes afluid drainage groove.

C6. The aircraft of paragraph C1, wherein the outer shock absorbercanister includes an elongated tube and an end fitting, the end fittingbeing configured to couple with one of a tail skid shoe and an airframe,where the end fitting includes an end fitting aperture through which theindicator rod extends.

C7. The aircraft of paragraph C6, wherein the end fitting includes adatum surface adjacent the end fitting aperture where the datum surfacedefines a measuring point from which a distance the indicator rodprotrudes from the end fitting aperture is determined.

C8. The aircraft of paragraph C6, wherein the indicator rod extendsthrough the end fitting aperture to provide a visual indicator of anamount of remaining energy absorption in the crushable indicatorcartridge.

C9. The aircraft of paragraph C6, wherein the elongated tube includes afirst tube portion and a second tube portion that are moveable relativeto each other along a longitudinal axis of the elongated tube.

C10. The aircraft of paragraph C6, wherein the end fitting is removablycoupled to the elongated tube so that removal of the end fitting fromthe elongated tube facilitates replacement of the crushable indicatorcartridge.

C11. The aircraft of paragraph C6, further comprising an anti-rotationmember removably coupled to both the elongated tube and the end fitting.

C12. The tail skid shock absorber of paragraph A6, wherein the at leastone end fitting aperture includes two end fitting aperturessymmetrically arranged on the end fitting.

C13. The aircraft of paragraph C1, wherein an outer diameter ofcrushable indicator cartridge is smaller than an inner diameter of theouter shock absorber canister.

C14. The aircraft of paragraph C1, wherein the indicator rod includes atleast one graduation that indicates an amount of remaining energyabsorption of the tail skid shock absorber.

D1. A method for indicating an amount of energy absorption remaining ina tail skid shock absorber, the method comprising:

inserting a crushable indicator cartridge into an outer shock absorbercanister, where the crushable indicator includes an indicator rodcoupled to the crushable indicator cartridge so as to move with aportion of the crushable indicator cartridge as a unit; and

crushing the crushable indicator cartridge through contact of theairplane tail skid with an object so that the indicator rod extends andis exposed from the outer shock absorber canister.

D2. The method of paragraph D1, where an amount of extension of theindicator rod relative to the outer shock absorber canister indicatesthe amount of energy absorption remaining.

D3. The method of paragraph D1, wherein the amount of extension isdetermined from a datum surface on an end fitting of the tail skid shockabsorber.

D4. The method of paragraph D1, wherein the crushable indicatorcartridge includes an elongated member that defines a crush core thatincludes a first plate coupled to the crush core, the indicator rodbeing coupled to the first plate, and a second plate coupled to thecrush core, the second plate defining an outer surface of the crushableindicator cartridge and including a second plate aperture through whichthe indicator rod extends, and wherein the first plate and the indicatorrod move as a unit during crushing of the crushable indicator cartridge.

D5. The method of paragraph D1, further comprising replacing a crushedcrushable indicator cartridge with an uncrushed crushable indicatorcartridge when a predetermined amount of energy has been absorbed by thetail skid shock absorber.

D6. The method of paragraph D1, wherein an amount of remaining energyabsorption of the tail skid shock absorber is indicated throughgraduations of the indicator rod.

D7. The method of paragraph D1, wherein an amount of remaining energyabsorption of the tail skid shock absorber is visually indicated by anamount of the indicator rod extending from the outer shock absorbercanister.

In the figures, referred to above, solid lines, if any, connectingvarious elements and/or components may represent mechanical, electrical,fluid, optical, electromagnetic, wireless and other couplings and/orcombinations thereof. As used herein, “coupled” means associateddirectly as well as indirectly. For example, a member A may be directlyassociated with a member B, or may be indirectly associated therewith,e.g., via another member C. It will be understood that not allrelationships among the various disclosed elements are necessarilyrepresented. Accordingly, couplings other than those depicted in thedrawings may also exist. Dashed lines, if any, connecting blocksdesignating the various elements and/or components represent couplingssimilar in function and purpose to those represented by solid lines;however, couplings represented by the dashed lines may either beselectively provided or may relate to alternative examples of thepresent disclosure. Likewise, elements and/or components, if any,represented with dashed lines, indicate alternative examples of thepresent disclosure. One or more elements shown in solid and/or dashedlines may be omitted from a particular example without departing fromthe scope of the present disclosure. Environmental elements, if any, arerepresented with dotted lines. Virtual (imaginary) elements may also beshown for clarity. Those skilled in the art will appreciate that some ofthe features illustrated in the figures, may be combined in various wayswithout the need to include other features described in the figures,other drawing figures, and/or the accompanying disclosure, even thoughsuch combination or combinations are not explicitly illustrated herein.Similarly, additional features not limited to the examples presented,may be combined with some or all of the features shown and describedherein.

In FIG. 8, referred to above, the blocks may represent operations and/orportions thereof and lines connecting the various blocks do not implyany particular order or dependency of the operations or portionsthereof. Blocks represented by dashed lines indicate alternativeoperations and/or portions thereof. Dashed lines, if any, connecting thevarious blocks represent alternative dependencies of the operations orportions thereof. It will be understood that not all dependencies amongthe various disclosed operations are necessarily represented. FIG. 8 andthe accompanying disclosure describing the operations of the method(s)set forth herein should not be interpreted as necessarily determining asequence in which the operations are to be performed. Rather, althoughone illustrative order is indicated, it is to be understood that thesequence of the operations may be modified when appropriate.Accordingly, certain operations may be performed in a different order orsimultaneously. Additionally, those skilled in the art will appreciatethat not all operations described need be performed.

In the foregoing description, numerous specific details are set forth toprovide a thorough understanding of the disclosed concepts, which may bepracticed without some or all of these particulars. In other instances,details of known devices and/or processes have been omitted to avoidunnecessarily obscuring the disclosure. While some concepts will bedescribed in conjunction with specific examples, it will be understoodthat these examples are not intended to be limiting.

Unless otherwise indicated, the terms “first,” “second,” etc. are usedherein merely as labels, and are not intended to impose ordinal,positional, or hierarchical requirements on the items to which theseterms refer. Moreover, reference to, e.g., a “second” item does notrequire or preclude the existence of, e.g., a “first” or lower-numbereditem, and/or, e.g., a “third” or higher-numbered item.

Reference herein to “one example” means that one or more feature,structure, or characteristic described in connection with the example isincluded in at least one implementation. The phrase “one example” invarious places in the specification may or may not be referring to thesame example.

As used herein, a system, apparatus, structure, article, element,component, or hardware “configured to” perform a specified function isindeed capable of performing the specified function without anyalteration, rather than merely having potential to perform the specifiedfunction after further modification. In other words, the system,apparatus, structure, article, element, component, or hardware“configured to” perform a specified function is specifically selected,created, implemented, utilized, programmed, and/or designed for thepurpose of performing the specified function. As used herein,“configured to” denotes existing characteristics of a system, apparatus,structure, article, element, component, or hardware which enable thesystem, apparatus, structure, article, element, component, or hardwareto perform the specified function without further modification. Forpurposes of this disclosure, a system, apparatus, structure, article,element, component, or hardware described as being “configured to”perform a particular function may additionally or alternatively bedescribed as being “adapted to” and/or as being “operative to” performthat function.

Different examples of the apparatus(es) and method(s) disclosed hereininclude a variety of components, features, and functionalities. Itshould be understood that the various examples of the apparatus(es) andmethod(s) disclosed herein may include any of the components, features,and functionalities of any of the other examples of the apparatus(es)and method(s) disclosed herein in any combination, and all of suchpossibilities are intended to be within the scope of the presentdisclosure.

Many modifications of examples set forth herein will come to mind to oneskilled in the art to which the present disclosure pertains having thebenefit of the teachings presented in the foregoing descriptions and theassociated drawings.

Therefore, it is to be understood that the present disclosure is not tobe limited to the specific examples illustrated and that modificationsand other examples are intended to be included within the scope of theappended claims. Moreover, although the foregoing description and theassociated drawings describe examples of the present disclosure in thecontext of certain illustrative combinations of elements and/orfunctions, it should be appreciated that different combinations ofelements and/or functions may be provided by alternative implementationswithout departing from the scope of the appended claims. Accordingly,parenthetical reference numerals in the appended claims, if any, arepresented for illustrative purposes only and are not intended to limitthe scope of the claimed subject matter to the specific examplesprovided in the present disclosure.

What is claimed is:
 1. An aircraft tail skid shock absorber comprising:an outer shock absorber canister configured to couple with one of a tailskid shoe or an airframe; a crushable indicator cartridge disposedwithin the outer shock absorber canister; and an indicator rod coupledto the crushable indicator cartridge so as to move with a portion of thecrushable indicator cartridge as a unit, where the indicator rod extendsthrough an aperture in a wall of the outer shock absorber canister,wherein the indicator rod includes at least one graduation thatindicates an amount of remaining energy absorption of the tail skidshock absorber.
 2. The aircraft tail skid shock absorber of claim 1,wherein the crushable indicator cartridge forms an elongated member thatdefines a crush core including: a first plate coupled to the crush core,the indicator rod being coupled to the first plate; and a second platecoupled to the crush core, the second plate defining an outer surface ofthe crushable indicator cartridge and including a second plate aperturethrough which the indicator rod extends, the second plate being separateand distinct from the outer shock absorber canister.
 3. The aircrafttail skid shock absorber of claim 2, wherein the indicator rod includesa rod head and the first plate includes a first plate aperture, theindicator rod extends through the first plate aperture where the rodhead cooperates with the first plate aperture so that the indicator rodis retained on the first plate.
 4. The aircraft tail skid shock absorberof claim 2, wherein the second plate includes fasteners configured tocouple the crushable indicator cartridge to an end fitting of the tailskid shock absorber.
 5. The aircraft tail skid shock absorber of claim1, wherein the indicator rod extends through the aperture in the wall ofthe outer shock absorber canister to provide a visual indicator of anamount of remaining energy absorption in the crushable indicatorcartridge.
 6. The aircraft tail skid shock absorber of claim 1, whereinthe outer shock absorber canister includes a first tube portion and asecond tube portion that are moveable relative to each other along alongitudinal axis of the outer shock absorber canister.
 7. The aircrafttail skid shock absorber of claim 1, wherein the outer shock absorbercanister includes a removable end fitting, where removal of the endfitting from the outer shock absorber canister facilitates replacementof the crushable indicator cartridge.
 8. A method for indicating anamount of energy absorption remaining in an aircraft tail skid shockabsorber, the method comprising: inserting a crushable indicatorcartridge into an outer shock absorber canister, where the crushableindicator cartridge includes an indicator rod coupled to the crushableindicator cartridge so as to move with a portion of the crushableindicator cartridge as a unit; and crushing the crushable indicatorcartridge through contact of the aircraft tail skid with an object sothat the indicator rod extends through an aperture in an end of theouter shock absorber canister and is exposed from the outer shockabsorber canister, where the end of the outer shock absorber canistercouples the tail skid shock absorber to one of a tail skid shoe and anairframe and the indicator rod includes at least one graduation thatindicates an amount of remaining energy absorption of the tail skidshock absorber.
 9. The method of claim 8, where an amount of extensionof the indicator rod relative to the outer shock absorber canisterindicates the amount of energy absorption remaining.
 10. The method ofclaim 8, wherein the crushable indicator cartridge includes an elongatedmember that defines a crush core that includes a first plate coupled tothe crush core, the indicator rod being coupled to the first plate, anda second plate coupled to the crush core, the second plate defining anouter surface of the crushable indicator cartridge and including asecond plate aperture through which the indicator rod extends, andwherein the first plate and the indicator rod move as a unit duringcrushing of the crushable indicator cartridge.
 11. The method of claim8, further comprising replacing a crushed crushable indicator cartridgewith an uncrushed crushable indicator cartridge when a predeterminedamount of energy has been absorbed by the tail skid shock absorber. 12.The method of claim 8, wherein an amount of remaining energy absorptionof the aircraft tail skid shock absorber is visually indicated by anamount of the indicator rod extending from the outer shock absorbercanister.
 13. An aircraft comprising: an airframe; and a tail skid shockabsorber coupled to the airframe, the tail skid shock absorberincluding: an outer shock absorber canister; a crushable indicatorcartridge disposed within the outer shock absorber canister; and anindicator rod coupled to the crushable indicator cartridge so as to movewith a portion of the crushable indicator cartridge as a unit, where theindicator rod extends through an aperture in a wall of the outer shockabsorber canister, wherein the indicator rod includes at least onegraduation that indicates an amount of remaining energy absorption ofthe tail skid shock absorber.
 14. The aircraft of claim 13, wherein thecrushable indicator cartridge forms an elongated member that defines acrush core including: a first plate coupled to the crush core, theindicator rod being coupled to the first plate; and a second platecoupled to the crush core, the second plate defining an outer surface ofthe crushable indicator cartridge and including a second plate aperturethrough which the indicator rod extends.
 15. The aircraft of claim 14,wherein the indicator rod includes a rod head and the first plateincludes a first plate aperture, the indicator rod extends through thefirst plate aperture where the rod head cooperates with the first plateaperture so that the indicator rod is retained on the first plate. 16.The aircraft of claim 14, wherein the second plate includes fastenersconfigured to couple the crushable indicator cartridge to an end fittingof the tail skid shock absorber.
 17. The aircraft of claim 14, whereinthe second plate includes a fluid drainage groove.
 18. The aircraft ofclaim 13, wherein the outer shock absorber canister includes anelongated tube and an end fitting, the end fitting being configured tocouple with one of a tail skid shoe and an airframe, where the endfitting includes an end fitting aperture through which the indicator rodextends.
 19. The aircraft of claim 18, wherein the end fitting includesa datum surface adjacent the end fitting aperture where the datumsurface defines a measuring point from which a distance the indicatorrod protrudes from the end fitting aperture is determined.
 20. Theaircraft of claim 18, further comprising an anti-rotation memberremovably coupled to both the elongated tube and the end fitting.